The invention relates to a new zeolite of the beta type and to a process for preparing this zeolite.
Because of their geometric selectivity and ion exchange properties, zeolites are utilized in industry on a large scale, in adsorption (for example gas drying, separation of linear and branched paraffins, separation of aromatic compounds, etc.) as well as in catalysis (for example catalytic cracking, hydrocracking, isomerization, oligomerization, etc.).
The chemical composition of the zeolites containing in their structure the elements Si and Al can be represented by the following approximate formula: EQU M.sub.2/n O,Al.sub.2 O.sub.3,xSiO.sub.2
where M represents a cation of valence n, such as for example an alkaline, an alkaline-earth or an organic cation,
x ranges, according to the structures, between 2 and infinity, in which case the zeolite is a microporous silica.
Although numerous zeolites of the aluminosilicate type do exist in nature, the search for products with new properties has led, the last few years, to the synthesis of a large variety of aluminosilicates of zeolitic structure. A new zeolite, without any known natural equivalent, which was discovered at the end of the seventies, is beta zeolite (U.S. Pat. No. 3,308,069, European Patent 64,328, U.S. Pat. No. 4,061,717). This zeolite is also known as NU-2 (European Patent 55,046). Zeolites of beta type are always syntheized in the presence of sodium cations and of a very limited number of organic structuring agents. In practice, in order to obtain a beta zeolite, it is necessary to start from a reaction mixture containing Na.sup.+ and TEA.sup.+ (tetraethylammonium) cations.
All the beta type zeolites which have been prepared presently have been synthetized in a conventional medium, that is to say in an alkaline medium with a pH value generally higher than 9, a medium in which the mobilizing agent of the silica is the OH.sup.- anion. Another synthesis medium of the zeolites has been discovered recently: it is the fluoride medium, in which the mobilizing agent of silica is the F.sup.- anion; in this medium, the pH value is generally lower than 10 (see for example J. L. GUTH, H. KESSLER and R. WEY, Proc. 7th Int. Zeolite Conf., Tokyo, Aug. 17-22 1986, p. 121). The synthesis of a limited number of zeolitic structures has already been successful in this new medium, as for example MFI (French Patent Application 88/09,631) and ferrierite (French Patent Application 86/16,362).
In relation to the alkaline synthesis medium (OH.sup.-), the fluoride medium shows a certain number of very appreciable advantages. In fact, in an alkaline medium, most synthetized zeolites are metastable: more stable solid phases are therefore likely to appear during the synthesis and unwanted phases may be precipitated. This difficulty increases when large amounts are to be prepared going from the laboratory stage to the industrial stage. Further, the metastable zeolites in the basic reaction medium are obtained only through a strong supersaturation of active species in the medium. This causes a rapid nucleation and consequently leads to small size crystals, the average dimensions of these crystals ranging around one micrometer. Developping crystals with a larger size is therefore difficult in a basic medium. But, in certain applications, it may be desirable to have crystals with a larger size in order to preserve for example the thermal stability of the solid.
Numerous applications, especially in acid catalysis, require zeolites in a proton form and totally free of alkali metal or alkaline-earth metal compensation cations introduced during the synthesis. The proton form can be obtained by carrying out long and repeated ion exchanges with NH.sub.4 + cations for example, followed by calcining in order to decompose these cations into protons. This ion exchange stage could be avoided if it were possible to totally replace the alkali metal or alkaline-earth metal cations with cations decomposable during the synthesis, that is to say NH.sub.4 + and/or organic cations. It is impossible to introduce NH.sub.4 + cations into the solid during the synthesis in a basic medium because the pH value is too high and NH.sub.4 + would then be converted into NH.sub.3. Further, syntheses achieved with pH values where the NH.sub.4 + cation is stable are long and difficult because of the poor solubility of the silica sources at such low pH values.
Another advantage of the syntheses achieved in a fluoride medium, in relation to those carried out in a conventional OH.sup.- medium, involves producing solids having acid and ion exchange properties of different nature. The acid catalysts prepared from the solids obtained in a fluoride medium show improved catalytic properties. At this point, it is very important to see that the crystallographic structure of a solid is not sufficient to entirely define its properties particularly the acid properties which play an essential part in catalysis.
Unlike to their homologs prepared according to prior art techniques, beta type zeolites prepared according to the invention contain fluorine after the synthesis stage and also after the removal of the organic compounds introduced during the synthesis. Fluorine, as we shall see further on, gives the beta zeolites according to the invention particular acid and ion exchange properties.
Another important advantage of the fluoride synthesis medium is that it allows obtaining a beta zeolite not only free of sodium cations, but also from the TEA.sup.+ cation. This cation is usually introduced into the alkaline synthesis medium by means of the TEAOH base, which is an excessively costly reagent. The possibility, by changing the medium, of preparing a beta zeolite while avoiding this very expensive organic structurer is a positive point.